1. Field of the Invention
This invention relates to the field of polyesters and, more particularly, to processes for preparing polybutylene terephthalate by direct esterification of terephthalic acid and 1,4-butanediol.
2. Description of the Prior Art
One route to the synthesis of polybutylene terephthalate involves carrying out the direct catalytic esterification of terephthalic acid and 1,4-butanediol in two distinct stages: in the first stage, terephthalic acid and 1,4-butanediol are reacted in the presence of catalyst to form bis-hydroxybutyl terephthalate and oligomers thereof and in the second stage, the bis-hydroxybutyl terephthalate and the oligomers undergo polycondensation in the presence of catalyst to provide product polybutylene terephthalate. The first stage is considered at an end when the reaction medium becomes homogeneous, i.e., when the "clearing point" indicating the conversion of substantially all the terephthalic acid (which is highly insoluble in the reaction medium) has been reached. In conventional practice, the second stage is commenced (by increasing the temperature and applying a vacuum) only after the clearing point of the first stage reaction has occurred. Versions of the polyesterification procedure are disclosed in U.S. Pat. Nos. 3,050,533 and 3,185,668. Such a synthesis of polybutylene terephthalate, desirable as it may be in providing a direct route to this commercially important resin, is subject to the major disadvantage that terephthalic acid acts as a catalyst for the conversion of 1,4-butanediol to tetrahydrofuran thereby reducing yield and increasing the production cost of the resin. It has been proposed in U.S. Pat. No. 3,936,421 to minimize tetrahydrofuran production by esterifying terephthalic acid with 1,4-butanediol in the presence of a mixture of an organotin compound and an organotitanium compound as catalyst. Another proposal to minimize tetrahydrofuran production is made in U.S. Pat. No. 4,014,858 which teaches the use of a tetravalent tin catalyst having one organo-to-tin linkage.
Because of the foregoing disadvantage associated with the direct esterification method, the principal process of manufacturing polybutylene terephthalate remains the transesterification of a dialkyl terephthalate, generally dimethyl terephthalate, with 1,4-butanediol since less tetrahydrofuran is encountered in this procedure. However, transesterification remains an indirect route to polybutylene terephthalate and were it not for the fact that the direct synthesis approach produces unacceptably large quantities of tetrahydrofuran, the latter would clearly be the process of choice.